Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
There has been increased demand for sweet onions grown both for local sales and wholesale markets. There is also need for onions for the fresh cut (diced or sliced) market.
Success with onions starts with growing or obtaining transplants of adapted varieties. Direct seeded bulb onions do not perform well in spring plantings. Transplants are started in January in 200-288 cell flats in the greenhouse for March transplanting. Growers can also arrange to have transplant growers in the Southwest (Texas, Arizona) produce transplants and ship them to our area for spring planting. While it is too late to have plants grown for 2015, some transplant growers do produce surplus for sale.
Onions also benefit greatly by being transplanted into black plastic mulch. Four foot wide plastic is laid on a raised bed such that there is a 3 foot bed top with 2 drip tapes. Four rows of onions can then be planted in the bed, with 8-10 inches between rows and 4-6 inches between transplants and a drip tape between each pair of rows.
Planting date impacts yield and bulb size, so it is very important to transplant at the right time. For sweet onions large (Jumbo) and colossal sizes greater than 3 inches in diameter have the most value. To achieve these sizes consistently, it is necessary to plant by the end of March. The later you plant in April, the lower the yields and sizes obtained.
Sweet onions have low pungency which is determined by measuring pyruvate and must have a score of 5.0 µmol/gfw or less, using a standard onion pungency test, to be marketed as a sweet onion.
In general, intermediate day sweet Spanish onion types are best adapted for our area; however, some long day varieties also can be grown successfully. The standard yellow sweet onion variety has been ‘Candy’. Other recommended yellow varieties are Spanish Medallion, Expression, and Exacta. Some yellow varieties for trial include Cimarron, Great Western, Salute, and Avalon. White onion varieties recommended include Super Star and Mt. Whitney. Recommended red varieties are Red Sky and Red Wing.
It is important that once transplanted, onion growth is not interrupted. Steady, frequent applications of irrigation water are necessary because onions have small root systems. If beds are allowed to dry out at any time, yields will be reduced. Fertility varies with grower and field but in general 50 lbs. of N/acre is applied preplant along with P and K based on a soil test. An additional 25-50 lbs. N/acre is applied through drip before bulbing starts.